OK, well, there's been a bit of banter about power supplies lately, so I figured I'd start something here with the hopes of turning it into a complete document at some point.

Power supplies are one of those silly things - a good power supply won't add anything to your sound, but a bad power supply can really mess things up!

First, let's talk about different types of power supplies. I used to work with tube amplifiers (still have a blackface Showman and a 70's Ampeg SVT!). Tube amps use quite high voltages, maybe 400V or higher. The power supplies are unregulated, meaning that the output from the supply isn't controlled or limited. If the mains voltage goes up or down, the PSU output will go up or down proportionately. Because of this, you may find that your tube amp sounds great in one place but not another. If you don't have enough power, you won't drive your tubes enough, and if you have too much power you might burn them out. One solution to this is to carry a variac around with you and adjust it accordingly...

In a similar manner to controlling your audio levels, you can limit, or regulate, the output voltage from a power supply. There are different kinds of regulators (linear, switching), but they all smooth out the power signal and limit it to a certain voltage. Linear regulators adjust their resistance based upon load and provide a constant output voltage. Because of this resistance, heat is generated inside the linear regulator and must be dissipated. Switching regulators are similar to using PWM to control the brightness of an LED. That is, a switcher turns on and off to control its output. Switchers are much more efficient and there is very little heat generated.

You'll probably hear purists denigrate switching regulators. If you use an older SMPS (switch mode power supply) that has a low switching rate in the audio range, it can really mess up your sound! Newer switchers operate in the MHz range, which won't interfere with your audio at all.

More soon...

Regards,
Doug_________________Once you start down the modular path, forever will it dominate your destiny!

OK, I know it's tempting to want to dive right in and solder something together, but we have to decide what we're trying to accomplish first.

For the purposes of this discussion, let's set some goals. First, the output of our supply: there are a lot of DIY analog circuit designs that use both positive and negative voltages. Both 12V and 15V designs are quite common. I've chosen 15V since I work primarily with 5U and 3U (Frac) sizes.

Next, we have to take into account what our input is. I'm located in the United States, so I deal with 120V AC mains.

After we get the power set up for our analog modules, then we'll figure out how to run our digital side. The chips I use operate on 5V, so at the end of this example, we'll have a power supply that provides +15V, -15V and +5V.

Regards,
Doug_________________Once you start down the modular path, forever will it dominate your destiny!

Most power ratings are just telling you what the maximum possible is allowed to be. For example, if you select a capacitor with a 35V rating, that means it will work with any voltage up to 35V. If you select a regulator that is rated at 1A, that means that you can draw up to 1A out of that regulator. It doesn't take 35V to "run" that capacitor, or 1A to run that regulator...

Regards,
Doug_________________Once you start down the modular path, forever will it dominate your destiny!

The supply in that article only creates a positive voltage. (Or I suppose, to be pedantic, it only allows positive voltages through.) To create a negative supply, simply reverse the directions of the rectifiers. If you need both positive and negative voltages, we combine the two forward rectifiers with the two reverse rectifiers. You can see an example of that in this CGS power supply diagram:

Something I always find a PITA is the math behind a basic 78xx/79xx supply.

I know that having raw DC enter the reg at 3 volts above (for positive) the regulator's output is ideal to keep heat dissipation to a minimum.

We should have formulae posted here I think to help with that. For example, if I want a +/- 12 volts supply (1 ampere each), how many volts should the transformer output - for halfwave and also for fullwave.

How large should I make the caps? (again, for both half and full wave rectifiers)... I know too large can cause an inrush problem, but too small allows possibly excess ripple. So we want it "just right".

I admit that I use a sort of rough guesstimate - and it has worked for me, but there's got to be formulae that we can use to make sure we buy the correct value stuff the first time._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

Sorry, I was going to post more before now, but I had a department meeting to go to. Day jobs, heh...

Anyway, yes, those two diodes are about protecting the regulator output. As I said, "paranoia" diodes, you can take that two ways, 1) if you're paranoid that you'll damage something it's a good idea to put them in, but 2) if you're paranoid enough to make sure that everything is exactly perfect, you won't need them...

D41 is a simple one, without it, the 5V section is separate and needs its own transformer, and with it, the 5V section takes its input from the same place as the +15V. I could have used a jumper, but I'm a bit paranoid... I had some transformers made up that have three secondary windings, two for 15V and one for 5V, so I leave that one out of my builds. For extra points, what about C21 and C26? Are they necessary if D41 is in place?

While I'm at it, C211 through C237 are distributed across the MB-808 PCB. (There are eleven separate voice sections.)

Regards,
Doug_________________Once you start down the modular path, forever will it dominate your destiny!

Could you clarify DGND a bit too? People will break their heads over that one once they built I guess ... as it goes "nowhere" now.

In this particular design, everything is included on a single board, so DGND (digital ground) and PE (analog ground) are planes that are poured copper on the PCB. L1 is a ferrite bead that connects the two. The two and three pin output headers are optional...

Regards,
Doug_________________Once you start down the modular path, forever will it dominate your destiny!

Looks like we're going to reach critical mass for power supply discussions here on E-M...

I had thought that this tip comes a few decades too late, as virtually nobody seems to build their own analogue supplies these days..

In an era when packets of peanuts are emblazoned with the statutory Health and Safety banner: "Warning: contains nuts", old-timers sometimes forget that some people have not yet learnt that electricity is dangerous. An earthed metal case for a power supply not only reduces the fire risk and protects you from exploding capacitors (!), but also cuts-down on hum pickup in amplifiers, shields fingers from lethal voltages and guards against the line/mains wire coming loose. When constructing the PSU, an earthed metal plate (perhaps the heat-sink for the bridge rectifiers) can be situated to shield the "hot" side of the transformer. A rubber grommet in the metal case protects the input supply lead and fitting a tie-wrap or simply tying a knot in the cable protects against bare wires being jerked free.

The end result can be much safer than cheap imported light-weight supplies in plastic boxes which radiate and in which the critical capacitor can break down and the box melt. Now we've got the legal stuff out of the way, let's get to the point...

Conventionally, one output of the bridge rectifier is grounded, or in a dual ± supply the centre-tap of a 12-0-12 transformer is grounded. For a heavy-duty supply requiring heat-sinks, this requires insulating the regulator/output transistor from the grounded heatsink (messy and awkward), else risking sparks flying by having the heat-sink at the full "raw" rectified voltage.

If you choose a transformer with separate 0-12V, 0-12V outputs, you are free to re-locate the ground. In the attached diagram, the output power transistor is mounted directly on a grounded heat-sink and the "regulated output" comes straight from the bridge rectifier!

The diode completes a "poor-man's long-tailed pair" with the advantage that it can tolerate a large reverse-voltage. If an accidental output load is your screwdriver to ground, the diode cuts-off and current is limited to the current through the emitter resistor, multiplied by the gain of transistor Q2. This protects the components until the fuse blows.

The MJ3001 is a power Darlington, with built-in base-emitter resistor. Its TO3-style metal case is connected to the collector.

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